Corn plastic (bioplastic, polylactic acid, PLA)
The main claim to fame for the bioplastics is that they are "biodegradable" or "compostable". Unfortunately, according to the Smithsonian Magazine article That is not quite correct. There are circumstances in which the PLA will decompose into water and carbon dioxide, but wast majority of consumer products end up (as of now) in a landfill, and decompose just as fast (that is, slow), as any other plastic.
NatureWorks, LLC, claims that it uses 2.5 pounds of corn per 1 pound of PLA. In a paper published by Dr. Vink, (in "Polymer Degradation and Stability", 2003) we can find the following energy use data (starting from corn already delivered to the PLA factory):
For the total of 48.9 MJ/kg PLA, or 6.1 kWh/lb of PLA plastic.
We will assume that half of this energy comes from electricity, and half from burning natural gas for heat.